The present invention relates to voltage generators, and more particularly, to a low temperature-corrected, constant voltage generator device including a reference voltage generator of the band gap type.
Reference voltage generators and in particular those of the band gap type are not generally able to deliver high currents. In addition, the generators are frequently associated with an amplifier to form a device able to supply a higher current at a constant voltage. Such a device is diagrammatically illustrated in FIG. 1. In FIG. 1, reference numeral 10 designates a well known reference voltage generator of the band gap type, for example, as disclosed in FR-A-2 767 207 to the same inventors. The reference voltage generator 10 is connected between a ground terminal 22 and an amplifier input. In the case of FIG. 1, the amplifier 10 is connected to a noninverting input 24+ of an operational amplifier 26. Throughout the remainder of the text, the voltage delivered by the reference voltage generator 10 is designated VREF. The voltage delivered by the complete constant voltage generator device is available in an output terminal 28 of the amplifier and is designated VOUT.
A divider bridge 30 is formed by a first resistor 31 of value R1 in series with a second resistor 32 of value R2. It is connected between the output terminal 18 and the ground terminal 22. A node 34, between the first and second resistors, is connected to the noninverting input 24xe2x88x92 of amplifier 26, to deliver there the divided voltage as the feedback voltage.
The available output voltage VOUT is such that:                                           V            OUT                    =                                    V              REF                        ⁢                                                            R                  1                                +                                  R                  2                                                            R                2                                                    ⁢                  
                ⁢                              V            OUT                    =                                    V              REF                        ⁡                          (                              1                +                                                      R                    1                                                        R                    2                                                              )                                                          (                  equation          ⁢                      xe2x80x83                    ⁢                      (            1            )                          )            
The reference voltage generator 10 can be adjusted so that the voltage VREF is substantially constant with the temperature in a relatively wide temperature range. However, it is found that the reference voltage value VREF has a linearity fault or error due to a second order term of the temperature development, which is characterized by a so-called bell-shaped temperature behavior. This behavior is illustrated in FIG. 2, which indicates in an arbitrary scale, the value of the voltage VREF on the ordinate, as a function of the temperature on the abscissa.
It can be seen that the bell-shaped behavior is particularly illustrated by a negative inflexion of the voltage curve for low temperatures. Such an inflexion also occurs for high temperatures.
FIG. 2 also indicates the value of the output voltage VOUT of the complete device which, to within a translation, reproduces the bell-shaped behavior of the voltage VREF. This behavior of the output voltage VOUT can be explained by the temperature identical evolution of the first and second resistors, which essentially have the same temperature coefficients. In other words, the ratio       R    2        R    1  
in the equation (1) remains constant no matter what the temperature.
The second order linearity error of the reference voltage generator (VREF) and the output voltage (VOUT) of the complete voltage generator device finally has repercussions on the equipment equipped with such a device and which are liable to operate, not only at ambient temperatures, but also in a low temperature range.
An object of the invention is to provide a voltage generator device for which the second order linearity error referred to above is corrected, particularly at low temperatures.
To achieve this an other objectives, the invention more specifically relates to a low temperature-corrected, constant voltage generator device comprising:
a reference voltage generator having a second order linearity error in a low temperature range,
an amplifier connected between the reference voltage generator and an output terminal,
a voltage divider, connected to the output terminal and connected to an input of the amplifier to supply the latter with a feedback voltage.
According to the invention, the voltage divider comprises at least one first resistor in series with an element having, at least in the low temperature range, an impedance with a temperature dependence behavior different from that of the first resistor, so as to supply a lower feedback in the low temperature range and a stronger feedback outside the range. The element in series with the first resistor can be a passive element, such as e.g. a second resistor having a temperature coefficient different from that of the first resistor.
According to another embodiment, the voltage divider can also have at least one active element, whose characteristic with the temperature is different from that of the resistor. As a result of these characteristics, the voltage divider produces a feedback voltage which varies with the temperature and which makes it possible to partly or totally correct the bell-shaped behavior of the reference generator.
The value of the first resistor of the divider, like the characteristics of the reference voltage generator, can be adjusted so as to obtain an optimum correction. In particular, the slope and consequently the term of the first temperature dependence order of the reference voltage supplied by the reference voltage generator can be adjusted in such a way that the reference voltage is constant with the temperature, to within the second and third order linearity errors. The first resistor of the voltage divider can have a value adjusted as a function of the second order error of the reference voltage generator, to obtain at the output terminal a voltage which is quasi-constant with the temperature (only the third order remains).
According to an embodiment of the device in which the element in series with the first resistor is active, the latter can incorporate one or more bipolar transistors. The transistor or transistors are then connected in series with the first resistor of the voltage divider by the collector and emitter terminals. They are also polarized or biased to operate under saturation conditions for temperatures equal to or higher than-the temperatures of the low temperature range.
Although the active element can have several transistors in chain or parallel form, the following description relates to only one of these transistors for simplification reasons. The nonlinear temperature character of the bipolar transistor of which advantage is taken in the aforementioned embodiment is due to the fact that a bipolar transistor supplied with a constant collector current has a higher saturation when its operating temperature is higher. Ideally, the transistor can be polarized so as to be at the limit of the saturation conditions in the low temperature range and so as to be highly saturated when the temperature exceeds the low temperature range. The considered low temperature range is e.g. between xe2x88x9260 and +25xc2x0 C. Other temperature ranges can be taken into account by correspondingly modifying the polarization of the transistor.
The polarization of the bipolar transistor can, e.g. make use of a current source, which is connected to its base and which fixes its operating point. When several transistors are used as the nonlinear element, the bases of all these transistors can be controlled by the power supply.